(a) Field of the Invention
The present invention relates to control of a converter, and more particularly, to an apparatus and a method of controlling a large capacity converter used in a fuel cell vehicle.
(b) Description of the Related Art
Fuel cells convert chemical energy generated by oxidization of fuel into electrical energy. Although fuel cells and batteries both utilize a chemical cell which performs an oxidization and reduction reaction, in a battery, the cell reaction is generated inside a closed system, whereas in a fuel cell the fuel and oxidant (reactants) are continuously supplied from outside of the system and then are continuously removed to outside of the system again once the reaction has occurred. Recently, fuel cells have begun to be commercialized, and since a reaction product of the fuel cell is pure water, research related to the use of water as an energy source of an environmentally-friendly vehicle has been actively pursued.
In order to efficiently use energy, a fuel cell vehicle utilizes a hybrid system that has a secondary energy storing device, such as a battery or a super capacitor, in addition to the fuel cell, which is the primary energy source. Here, a bi-directional converter serves to maintain a uniform fuel utilization rate and balance power between the fuel cell and a load by charging and discharging the appropriate amount of energy through an energy storing device, such as a battery. Conventionally, a large capacity bi-directional converter is typically used to control the voltage of a direct current terminal, but when an inverter connected to a motor is regeneratively braked, the bi-directional converter operates in a hybrid control mode, in which output current and power are controlled together or simultaneously, in order to prevent overcurrent failure to from being generated due to a rapid change in the current.
In order to control the converter, a pulse width modulation (PWM) control method using a dedicated chip is typically used. This chip, however, is a PWM generation chip that is appropriate for a small or medium power converter. Thus, the maximum value of a triangular wave for generating the PWM is determined by the size of the chip, and thus the size of the maximum value is limited as a result. Accordingly, there is a limit to the amount of control offer for a large capacity converter, such as a fuel cell system.
FIG. 1 is a graph illustrating a triangular wave generated in a dedicated chip for controlling PWM in the related art. As illustrated in FIG. 1, when the dedicated chip is used, a maximum value of the triangular wave is fixed at approximately 2.3 V. Since maximum value of the triangular wave is limited and thus the amount of control offered, resolution and the degree of level precision of the overall control deteriorate as well. Additionally, the dynamic nature of the system is degraded.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.